The use of Roots type positive displacement blowers to supply charging air to the cylinders of two-stroke cycle diesel engines and the like has been well known for many years. Further it is known to provide such engines with an exhaust driven turbocharger which increases the charge at the high end of the engine operating load range by compressing air supplied to the blower inlet before it is forced by the blower into the engine cylinders, with or without cooling.
The prior art also proposes various arrangements for improving operating efficiency of such systems through bypassing of a portion of the charging air around the blower. Such bypassing may be used either in the reverse direction to reduce the blower pumping load or in the direction of flow to reduce the restriction of the blower to the supplying of additional charging air by the turbocharger. Such arrangements have added varying degrees of complexity to the blower or the associated intake air supply system and have, in some cases, been controlled by the different pressure across the blower.